Chemotherapy refers to the use of cytotoxic agents such as, but not limited to, cyclophosphamide, doxorubicin, daunorubicin, vinblastine, vincristine, bleomycin, etoposide, topotecan, irinotecan, taxotere, taxol, 5-fluorouracil, methotrexate, gemcitabine, cisplatin, carboplatin or chlorambucil in order to eradicate cancer cells and tumors. However, these agents are non-specific and, particularly at high doses, they are toxic to normal and rapidly dividing cells. This often leads to various side effects in patients undergoing chemotherapy and radiation therapy. Myelosuppression, a severe reduction of blood cell production in bone marrow, is one such side effect. It is characterized by leukopenia, neutropenia and thrombocytopenia. Severe chronic neutropenia (idiopathic, cyclic, and congenital) is also characterized by a selective decrease in the number of circulating neutrophils and an enhanced susceptibility to bacterial infections.
The essence of treating cancer with chemotherapeutic drugs is to combine a mechanism of cytotoxicity with a mechanism of selectivity for highly proliferating tumor cells over host cells. However, it is rare for chemotherapeutic drugs to have such selectivity. The cytotoxicity of chemotherapeutic agents limits administrable doses, affects treatment cycles and seriously jeopardizes the quality of life of the oncologic patient.
Although other normal tissues may also be adversely affected, bone marrow is particularly sensitive to proliferation-specific treatments such as chemotherapy or radiation therapy. Acute and chronic bone marrow toxicity is a common side effect of cancer therapies which leads to decreases in blood cell counts and anemia, leukopenia, neutropenia, agranulocytosis and thrombocytopenia. One cause of such effects is a decrease in the number of hematopoietic cells (e.g., pluripotent stem cells and other progenitor cells) caused by both a lethal effect of cytotoxic agents or radiation on these cells and by differentiation of stem cells provoked by a feed-back mechanism induced by the depletion of more mature marrow compartments. The second cause is a reduction in self-renewal capacity of stem cells, which is also related to both direct (mutation) and indirect (aging of stem cell population) effects. (Tubiana, M., et al., Radiotherapy and Oncology 29:1-17, 1993). Thus, cancer treatments often result in a decrease in Polymorphonuclear Neutrophils (PMN) or neutropenia. PMN are the first line of defense against invading pathogens and play a central role during acute inflammation, their primary function being the phagocytosis and killing of the infectious agents. To accomplish this role, PMN leave the circulation in response to chemotactic factors and enter in the affected area to exert their biological functions. In individuals exhibiting normal blood cell counts, neutrophils constitute approximately 60% of the total leukocytes. (SI Units Conversion Guide, 66-67 (1992), New England Journal of Medicine Books). However, as many as one in three patients receiving chemotherapy treatment for cancer may suffer from neutropenia. Mean normal neutrophil counts for healthy human adults are on the order of 4400 cells/μL, with a range of 1800-7700 cells/μL. A count of 1,000 cells to 500 cells/μL is moderate neutropenia and a count of 500 cells/μL or less is severe neutropenia. Patients in myelosuppressive states are prone to infection and frequently suffer from blood-clotting disorders, requiring hospitalization. Lack of neutrophils and platelets is the leading cause of morbidity and mortality following cancer treatments and contributes to the high cost of cancer therapy. In these above-mentioned conditions, the use of any agent capable of inhibiting neutrophil apoptosis or stimulating neutrophil activation and mobilization can be of therapeutic value. Efforts to restore the patient's immune system after chemotherapy involves the use of hematopoietic growth factors to stimulate remaining stem cells to proliferate and differentiate into mature infection fighting cells.
In bone marrow transplantation, a phenomenon known as “mobilization” has also been exploited to harvest greater numbers of stem/progenitor cells from peripheral blood. This method is currently used for autologous or allogeneic bone marrow transplantation. Growth factors are used to increase the number of peripheral progenitor stem cells to be harvested before myeloablative therapy and infusion of progenitor stem cells.
Post-therapy bone marrow transplantation can also counter neutropenia. However, these treatments require 10-15 days of treatment which leaves patients vulnerable to infection. Agents capable of stimulating bone marrow stem cells can facilitate and accelerate stem cells engraftment thus shortening the neutropenic window following bone marrow transplantation.
Although hematopoietic growth factors such as granulocyte-macrophage colony stimulating factor (GM-CSF) and granulocyte colony stimulating factor (G-CSF) can exert such actions, their use is expensive since they have to be produced by recombinant technology. Such post-therapeutic ameliorative treatments are unnecessary if patients are “chemoprotected” from immune suppression.
Therefore, there is a need for novel compositions and methods to reduce the undesirable side effects of myelosuppressive states induced by chemotherapy and radiation therapy.